1. Field of the Invention
The present invention generally relates to a magnetic actuator and a light quantity adjusting device including the magnetic actuator, and more particularly, to a diaphragm driving device used for controlling a lens aperture in accordance with a received light quantity in a video camera, a digital camera or the like.
2. Description of the Related Art
Aperture blades serve as a shading member of a light quantity adjusting device used for controlling a lens aperture in accordance with a received light quantity in a video camera, a digital camera or the like and are typically configured such that the blades are arranged in a pair and relatively moved to adjust the light quantity.
An exemplary magnetic actuator for actuating aperture blades is shown in FIG. 8. The actuator includes a coil bobbin (which consists of two portions divided in a vertical direction, an upper bobbin 101a and a lower bobbin 101b) 101 for winding a coil (not shown) thereon, the coil wound around the bobbin 101, an actuating lever 102 for actuating aperture blades (not shown), a rotor magnet 103 fixed to the actuating lever 102 for rotating the actuating lever 102, and a tubular yoke 104 for forming a path along which magnetic flux produced by the passage of electric current through the coil passes, that is, a magnetic path. The rotor magnet 103, fixed to the actuating lever 102, and the bobbin 101, around which the coil is wound, are housed in the yoke 104 (see Japanese Patent Laid Open No. H8(1996)-019239).
In the magnetic actuator, torque of the rotor magnet 103 produced by the passage of electric current through the coil is transmitted via the actuating lever 102 to a pair of aperture blades, not shown, as an actuating force therefor. The pair of aperture blades is slidably supported on a base plate, not shown, having a light passing port formed therein. Engaging pins 102a formed at both ends of the actuating lever 102 are fitted into through-holes formed in the respective aperture blades. The actuating lever 102 is rotated to drive the pair of aperture blades in parallel in opposite directions to change the area of the opening of the light passing port.
In a light quantity adjusting device with a magnetic actuator of the abovementioned type, the magnetic bias in the yoke 104 is used to maintain the pair of aperture blades serving as the shading member at a totally closed position after the power is shut off. As shown in FIG. 8, the magnetic bias of magnetic flux is provided by a slit formed in the yoke 104.
FIGS. 7(a) to 7(c) show the a diaphragm driving device in which the rotor magnet 103 is urged in one rotation direction by the magnetic bias of magnetic flux produced in end faces forming the slit 105 provided in the stator yoke 104.
Since the magnetic flux readily passes through the yoke 104, magnetic paths are formed as shown by arrows in FIGS. 7(a) to 7(c). The magnetically stablest state in the yoke 104 is achieved when a line P-P′ connecting the center of the yoke 104 with the center of the slit 105 coincides with the boundary between the poles of the rotor magnet 103. The position closest to the coincident state is defined as a totally closed position shown in FIG. 7(a). At the totally closed position, the magnetic bias provides a force in a counterclockwise direction (close direction) shown by an arc-shaped arrow in order to hold the shading member totally closed.
When electric current is passed through the coil, a magnetic circuit is formed from the N pole through the one of the end surfaces of the slit 105 (an intermediate position shown in FIG. 7(b)), and the rotor magnet 103 is rotated in a clockwise direction in accordance with the passed electric current against the magnetic bias force. When the passed electric current is increased, the rotor magnet 103 is rotated to a totally opened position shown in FIG. 7(c).
In Laid-open Japanese Utility Model Registration Application No. H7 (1995)-019740, a magnetic actuator is configured such that a rotor magnet is rotated in one direction and held at a predetermined position by using a magnetic bias force when electric current through a coil is shut off. A magnetic substance (pin) is included separately from a yoke on the outer circumference of a bobbin or on the inner circumference of the yoke to provide magnetic bias similar to that in Japanese Patent Laid-Open No. H8 (1996)-019239 described above, thereby attaining the similar effects.
In the magnetic actuator with the stator yoke (referred to as a slit yoke) having the slit disclosed in the abovementioned Japanese Patent Laid-Open No. H8(1996)-019239, however, the stator yoke has a low radial crushing strength and thus the yoke is prone to deformation if an external force is applied thereto. When a number of manufactured yokes having slits are stored n one place, an edge of a slit yoke may enter the slit portion of an adjacent yoke, and the yokes may get snagged on each other and may be deformed when a user pulls them out. In addition, the slit of the yoke tends to cause magnetic saturation more quickly than in a yoke having no slit.
The yoke is also prone to deformation and damage in assembly of the magnetic actuator, which may degrade the magnetic property of the magnetic circuit formed of the stator yoke and the magnet to reduce the driving torque. In this case, the shading member may not be closed completely and thus the light may not be blocked perfectly.
In the magnetic actuator with the slit yoke described above, after the bobbin is put into the yoke, the slit needs to be placed at an appropriate position through fine adjustments to cause the actuating lever to be in the totally closed state and then the yoke should be bonded to the bobbin with an adhesive or the like.
When a magnetic substance different from the yoke is provided on the inner circumference of the yoke or on the outer circumference of the bobbin as disclosed in Laid-open Japanese Utility Model Registration Application No. H7 (1995)-019740, the magnetic substance needs to be bonded to the yoke or the bobbin at an appropriate position to cause the actuating lever to be in the totally closed state. This results in an additional number of parts and an extra cost due to an increase in the number of manhours required for manufacture.
The yoke has a slight irregular magnetic bias even in a cylindrical shape. Even when the magnetic substance is bonded at the appropriate position, the abovementioned characteristic disadvantage occurs if the yoke is rotated later. To prevent this, the yoke should be fixed with an adhesive or the like to prevent a change in position of the yoke relative to the magnetic substance, which may take extra time and effort.